Steel cord wires and bead wires which have generally been used in tires and similar products are twisted strands made by twisting a bundle of filaments of a high carbon steel, each steel filament having a diameter of around 0.2 mm. Steel filaments which are presently used for this purpose have a tensile strength on the order of 320 kgf/mm.sup.2.
The conventional process for manufacturing such steel filaments comprises the following steps: ##STR1##
In the final lead patenting (LP) step, a 1.2.phi. steel wire is heated to about 900.degree. C. and then dipped in a molten lead bath at around 600.degree. C. to adjust the tensile strength (TS) of the wire to 125 kgf/mm.sup.2. The resulting lead-patented steel wire is used as a starting material for the final drawing, and it is pickled and plated before it is finally drawn into a filament having a tensile strength of about 320 kgf/mm.sup.2. In the above-described process, the wire drawing reduction ratio (.epsilon.) attained under these conditions is around 3.2. A higher reduction ratio is desired in order to improve the strength of the wire, but it cannot be attained due to a decrease in ductility.
In co-pending Japanese Patent Application No. 63-169480 (1988), the present inventors proposed that wire drawability can be increased by performing the final lead patenting under such conditions that the resulting wire has a relatively low tensile strength (TS) of around 115 kgf/mm.sup.2. However, the wire drawing reduction ratio (.epsilon.) attainable in this method is at most .epsilon.=4.5, and the tensile strength of the resulting filaments is on the order of 380 kgf/mm.sup.2.
In the process described in Japanese Unexamined Patent Application Kokai No. 64-15322(1989), a thermo-mechanical treatment is applied in place of the final lead patenting treatment so as to refine the resulting pearlite blocks to an average size of about 6-77 .mu.m and improve the wire drawability of the wire. This process gives steel filaments having a tensile strength on the order of 400 kgf/mm.sup.2. However, after the thermo-mechanical treatment, the wire is subjected to recrystallization by heating again at a temperature in the austenitic range followed by slow cooling. Therefore, the refinement of the pearlite blocks cannot be achieved in a stable manner, and the process involves an increased number of steps, thereby requiring a prolonged processing period and leading to increased manufacturing costs. Moreover, the reduction of area of the steel filaments obtained after the final wire drawing is on the order of 30% which is rather low since the working has been applied in a high reduction ratio region. Therefore, the resulting filaments lack stability and are susceptible to breakage during twisting into cord wires.
Japanese Examined Patent Publication No. 57-19168(1982) which corresponds to Japanese Unexamined Patent Application Kokai No. 53-30917(1978) discloses a similar strengthening or toughening method of a carbon steel by a thermo-mechanical treatment. The steel material obtained in this method is a steel rod having a diameter of from 4.0 mm to 13.0 mm and it is used in the as-treated state without further wire drawing. The thermo-mechanical treatment employed in this method is performed by applying working with a reduction of area in the range of 10% to 40% to a metastable austenitic structure at a relatively low temperature (which is below 450.degree. C. and above the Ms point) followed by isothermal heat treatment to form a structure comprising fine ferrite and cementite phases. In this case, the refinement attained by the thermo mechanical treatment is a reduction of interlaminar distance, i.e., lamellar distance, of the pearlite structure. This publication does not refer to a reduction of the pearlite block size as described above. The strength attained by the thermo-mechanical treatment is not higher than 200 kgf/mm.sup.2.
It is possible to increase the strength of a starting wire which is subjected to final drawing by increasing its carbon content to 1.0% or more, for example. However, the drawability of this material is degraded by the effect of precipitated proeutectoid cementite, and therefore the resulting drawn wire cannot have an improved tensile strength.
Nowadays, tire cord wires are required to have an even higher tensile strength as the properties required for tires become more strict in order to improve the stability of automobiles during high speed driving. Accordingly, steel filaments for use in the manufacture of tire cord wires are required to have improved mechanical properties after final wire drawing such as a tensile strength (TS) of at least 400 kgf/mm.sup.2 and a reduction of area of at least 40%.
In the manufacture of steel filaments, the tensile strength of the steel material is gradually increased in the course of drawing a starting wire of a high carbon steel to reduce the diameter. However, when a conventional starting steel wire having a diameter of 1-2 mm and containing a usual eutectoid structure is patented and then wire drawn, the maximum attainable tensile strength is around 320 kgf/mm.sup.2 with a reduction ratio .epsilon.=3.2, as described above.
Neither the above-mentioned technique of increasing the limiting reduction ratio .epsilon. by adjusting the structure so as to have relatively coarse grains before wire drawing or the technique of improving the drawability of the starting steel wire by refinement of grains (pearlite blocks) achieved by thermo-mechanical treatment as described in Japanese Unexamined Patent Application Kokai No. 64-15322(1989) can provide the desired steel filaments having a tensile strength of 400 kgf/mm.sup.2 or higher and a ductility of at least 40% by subsequent wire drawing of the starting wire.